Unfortunately, Voyager has not been updated for Mac OS X, so the following only
really applies if you are running an older system. Starry Night is an
excellent program for X and Equinox is a reasonable shareware astronomy
program.

Voyager is a well-designed, complete, fast astronomy tool. It is a program that
any amateur astronomer can use to plan an observing session or just to explore
the sky. It is also the most useful piece of software I have ever seen for
teaching astronomy. For example, imagine being able to teach the geometry of
eclipses by observing the same eclipse from different points on the Earth,
from the Moon and even from the Sun!

Depending on your requirements it can fit compactly on a hard disk without
consuming too much space, this makes it practical to run on a Powerbook and
use in the field. A full installation of all the star catalogs and graphics
takes a lot more space, of course.

Like all the programs I advocate on these pages I have no connection with the
manufacturers and recommend these products solely on how useful I found them
for work done for my clients and myself.